The present invention relates to a feed-forward amplifier, and more particularly to a feed-forward amplifier provided with a loop for detecting non-linear distortion of the main amplifier and a distortion eliminating loop for cancelling the detected distortion by synthesizing it into the output of the main amplifier.
Conventional linear amplifiers known to be used for simultaneous amplification of multiple frequencies in the high frequency band, among other purposes include a feed-forward amplifier disclosed in Japanese Patent Laid-open No. Hei 1(1989)-198809, for example. A typical configuration of this kind of feed-forward amplifier according to the prior art is illustrated in FIG. 6. In the feed-forward amplifier shown in this diagram, which has a distortion detecting loop 100 and a distortion eliminating loop 200, a main amplifier 4 collectively amplifies input multi-frequency multiplexed signals in the high frequency band entered through an input terminal 1. The distortion detecting loop 100 detects non-linear distortion components generating in the amplifying process by cancelling the entered signal components. The distortion eliminating loop 200 cancels the distortion components by amplifying the detected distortion components with an auxiliary amplifier 15 and again injecting the amplified distortion components into the output of the main amplifier 4.
The distortion detecting loop 100 consists of a pilot oscillator 18, a coupler (directional coupler) 19 for super-imposing a pilot signal over an input signal, a bi-divider 2, a vector adjuster 3 capable of adjusting the attenuation and phase shift quantities, a main amplifier 4, a delay line 6, a coupler 10, a detector 10 (DET) 22 and a control circuit 9, and is further equipped with couplers 7 and 8 for common use with the distortion eliminating loop 200 to be described in detail below. The output signal of the coupler 8 is detected by the detector 22 via the coupler 10, and fed to the control circuit 9, which controls the vector adjuster 3 so as to minimize the output level of the detector 22.
The aforementioned distortion eliminating loop 200 comprises the couplers 7 and 8 provided for common use with the above-described distortion detecting loop 100, a pilot oscillator 20, a coupler 21, a delay line 11, a coupler 12, a vector adjuster 13 capable of adjusting the attenuation and phase shift quantities, an auxiliary amplifier 15, a coupler 16, a detector (DET) 23 and a control circuit 14.
The control circuit 14 detects a pilot signal with the output circuit of the feed-forward circuit via the coupler 16 and the detector 23, and controls the vector adjuster 13 so as to minimize the detection level of the pilot signal.
The operation of the prior art feed-forward amplifier having such a configuration will be described below. Multi-frequency multiplexed signals in the high frequency band inputted to the input terminal 1, after being super-imposed by the coupler 19 over the output signal of the pilot oscillator 18, are bi-divided by the bi-divider 2, and one part of the signals resulting from the division, after undergoing the adjustment of its attenuation and phase shift quantities by the vector adjuster 3 and supplied to the main amplifier 4 to be amplified, is multiplexed by the coupler 21 with the pilot signal from the pilot oscillator 20, and entered into the coupler 12 via the coupler 7 and the delay line 11. These input signals to the coupler 12 are main amplifier signals, which have distortion components generated during amplification by the main generator 4.
The other part of the signals resulting from the division by the bi-divider 2, after being given an equal delay time by the delay line 6 to the signal delay time of the vector adjuster 3 and the main amplifier 4, is synthesized in a reverse phase by the coupler 8 with a part of the main amplifier signal branched by the coupler 7, and the synthesized signals are entered into the coupler 10. The pilot signal contained in the partial input signal branched by the coupler 10, after being detected by the detector 22, is supplied to the control circuit 9, which adjusts the attenuation and phase shift quantities of the vector adjuster 3 so as to minimize the output signal level of the detector 22.
Here, the only constituent element between the output end of the bi-divider 2 and the coupler 8 is the delay line 6, and the distortion arising on it can be ignored. Therefore, if the operation of the aforementioned distortion detecting group 100 is appropriate, the part of the input signals having passed the vector adjuster 3 and the main amplifier 4 and been amplified is synthesized in a reverse phase by the coupler 8 to cause only the distortion component generated or mixed mainly in the main amplifier 4 to be outputted from the coupler 8.
The distortion components outputted from the coupler 8 and inputted to the coupler 10, after undergoing adjustment of its attenuation and phase shift quantities by the vector adjuster 13, are supplied to the auxiliary amplifier 15 and amplified. Then, the amplified distortion components are synthesized in a reverse phase via the coupler 12 with the main amplifier signals, which have been delayed by the delay line 11 by the time length of propagation by the vector adjuster 13 and the auxiliary amplifier 15.
These synthesized signals are supplied to the coupler 16, where a part of them is branched and entered into the detector 23. The detector 23 extracts the pilot signal in the input signal by, for instance, synchronous detection, and supplies it to the control circuit 14. The control circuit 14 controls the attenuation and phase shift quantities of the vector adjuster 13 so as to minimize the output level of the detector 23. As a result, the main amplifier signals in which the pilot signal, i.e. the distortion components, have been minimized are 10 supplied to the output terminal 17.
The spectra in sections A through D are illustrated in FIGS. 7 (A) through (D), respectively, where f1 and f2 are input frequencies, and fx and fy, spurious responses.
The feed-forward amplifier disclosed in the Patent Laid-open No. 1989-198809 described so far cannot be considered to be always performing optimal control in terms of the input/output characteristics of the amplifier for the following reason.
FIG. 3 shows a model of the input/output characteristics of the main amplifier 4, the auxiliary amplifier 15 being assumed to be an ideal limiter amplifier whose output power is about 1/9 of that of the main amplifier. FIG. 4 shows a model of the input/ output characteristics of the auxiliary amplifier 15.
Hereupon is considered the nature of the signals that are inputted. FIGS. 8 and 9 illustrate examples of waveforms synthesized from eight sine waves. FIG. 8 shows a case in which the initial phase of every wave is zero and FIG. 9, another case in which the initial phases are set at random.
It is known that, when signals of N waves of an equal amplitude are synthesized, the peak power is N.sup.2 times the power per wave, but the power stays at this peak only for a moment.
FIG. 10 illustrates the distribution of instantaneous voltage when N sine waves of which the synthetic power is constant and the phases are not correlated (N=1, 2, 4, 8, 16, 32). Statistically, the distribution of instantaneous voltage when sine waves whose phases are not correlated are synthesized follows the normal distribution pattern. Therefore, when the number N of waves is great, the expected value of the input voltage at a given point of time is low. Accordingly if, as stated in the patent Laid-open No. 1989-198809, control is accomplished by cancelling the carried by superimposing the pilot signal over the input signals to set off the pilot signal, the distortion detecting loop is controlled so as to minimize the average power of the signals entered into the auxiliary amplifier.
As shown in FIG. 10, when many carriers are entered, the probability of the input voltage at a given point of time reaches its maximum in the vicinity of 0 V. Therefore, the distortion detecting loop is controlled so as to be established in the vicinity of 0 V input. For this reason, "the characteristic deemed to be free from distortion" in FIG. 3 is a tangent near the origin of "the input/output characteristic of the main amplifier" in FIG. 3. In order to achieve amplification without distortion, it is necessary to have the auxiliary amplifier compensate for deviation between the solid line (the input/output characteristic of the main amplifier) and the dotted line (the characteristic deemed to be free from distortion) in FIG. 3. The output of the auxiliary amplifier, as it is synthesized by a directional coupler with the signals from the main amplifier, needs to be greater correspondingly to that loss.
FIG. 3 shows the undistorted peak output voltage when the output of the auxiliary amplifier is synthesized by a -10 dB directional coupler and the saturated output power of the auxiliary amplifier is set to be about 1/9 of that of the main amplifier. FIG. 11 (A) shows the input voltage according to the conventional control method versus the voltage inputted to the auxiliary amplifier.
In a main amplifier to constitute a feed-forward amplifier, an operating point close to Class A is set in order to achieve a decent level of distortion characteristic even without compensation. Therefore, the gain decreases as the level of input signals rises. Consequently, the output of the auxiliary amplifier takes on a polarity to be added to that of the main amplifier. This means that only half of the available output range of the auxiliary amplifier is utilized. Accordingly, the control method according to the piror art involves the disadvantage of a narrow input range in which feed forward can normally function.